1. Field of the Invention
The present invention relates to a sulfated fucan oligosaccharide which is useful in a field of glycotechnology, and a method for producing the same.
2. Description of Related Art
Brown algae contain a variety of sulfated polysaccharides. These polysaccharides are often generically called fucoidans or fucoidins. In many cases, their structures vary depending on the algae from which they derive. For example, sulfated polysaccharides extracted from Fucus vesiculosus, Laminaria japonica, Cladosiphon okamuranus, Nemacystus decipiens and sporophyll of Undaria pinnatifida have structures different each other. Therefore, it is necessary to obtain enzymes that digest the respective sulfated polysaccharides in order to obtain oligosaccharides from the sulfated polysaccharides by enzymatically digesting them or to determine their structures.
Molecular species of sulfated polysaccharides including sulfated fucans, sulfated fucoglucuronomannans and sulfated fucogalactans as well as several other molecular species have been reported. Sulfated polysaccharides generally have some biological activities in many cases. For example, a sulfated fucan fraction has been reported to have a strong anticoagulant activity, and a sulfated fucoglucuronomannan fraction has been reported to have an apoptosis-inducing activity against tumor cells.
If a sulfated polysaccharide is to be developed as a pharmaceutical, it is necessary to determine its structure. It is very advantageous to determine the structure using an enzyme that digests the sulfated polysaccharide. However, no enzyme that digests a sulfated polysaccharide from a brown alga is commercially available. In addition, a digesting enzyme that specifically digests the sulfated polysaccharide of which the structure is to be determined is required. This is because sulfated polysaccharides from brown algae vary depending on the species of the algae in many cases. Structures of sulfated polysaccharides derived from algae belonging to Laminariales have been studied, although structures have been revealed only for a few kinds among many molecular species.
As described above, a structurally homogeneous sulfated fucan oligosaccharide which is produced by enzymatic means using an enzyme that digests a novel sulfated polysaccharide derived from an alga belonging to Laminariales (i.e., an enzyme that specifically digests a sulfated fucan) has been desired.
Thus, the main object of the present invention is to provide a smaller molecule obtainable by allowing a sulfated fucan-digesting enzyme which digests a novel sulfated polysaccharide derived from an alga belonging to Laminariales to act on a sulfated fucan, and a method for producing the same.
The first aspect of the present invention relates to a sulfated fucan having the following chemical and physical properties:
(1) containing fucose as a constituting saccharide;
(2) containing a sulfated saccharide of general formula (I) as an essential component of the constituting saccharide: 
wherein R is H or SO3H, at least one of Rs is SO3H and n is an integer of 1 or more; and
(3) being converted into smaller molecules by a sulfated fucan-digesting enzyme derived from Alteromonas sp. SN-1009 to generate at least one compound selected from the group consisting of the compounds of general formulas (II), (III), (XIII), (XIV), (XV) and (XVI): 
wherein R is H or SO3H and at least one of Rs is SO3H in all formulas above,
or a salt thereof.
The second aspect of the present invention relates to a sulfated fucan oligosaccharide of general formula (I): 
wherein R is H or SO3H, at least one of Rs is SO3H and n is 1 to 5.
The third aspect of the present invention relates to a method for preparing a sulfated fucan oligosaccharide, the method comprising:
allowing a sulfated fucan-digesting enzyme derived from Alteromonas sp. SN-1009 to act on a sulfated fucan of the first or second aspect; and
collecting a digestion product.
According to the third aspect, the sulfated fucan is derived from Kjellmaniella crassifolia, Laminaria japonica or Lessonia nigrescens. 
The fourth aspect of the present invention relates to a sulfated fucan oligosaccharide obtainable by the preparation method of the third aspect.
The fifth aspect of the present invention relates to a sulfated fucan oligosaccharide having a chemical structure selected from the group consisting of general formulas (II), (III), (XIII), (XIV), (XV) and (XVI): 
wherein R is H or SO3H and at least one of Rs is SO3H in all formulas above,
or a salt thereof.
As a result of intensive study, the present inventors have found a method for producing a sulfated fucan oligosaccharide by digesting a novel sulfated polysaccharide derived from a alga belonging to Laminariales using a sulfated fucan-digesting enzyme. The sulfated fucan oligosaccharide can be utilized as a reagent for glycotechnology and is structurally homogeneous. Furthermore, the present inventors have determined the structure of the oligosaccharide. Thus, the present invention has been completed.